Viewport 2D of 3D object and textureimage

Hello everybody,

Can some one help me please!

 I have created a viewport using Helix tool kid. I put camera at the position (0,0,0). then I create sky box around camera then show the part camera can view to view port.  My problem is when my view port is showing 1 part of image. How I can know where is the area showing in view port at 2D image that I used for texture to the cube?